Many separations have been found to be effected by selective permeation across a thin membrane and many types of polymers have been utilized as membranes. Flat membranes as closures of cells for dialysis, reverse osmosis, and other separation processes in general have been found inefficient or expensive because of size of membranes, lack of strength, pressures withstood and the like. To meet some of these problems hollow fiber membranes have been developed, but many materials from which such hollow fibers can be spun also suffer from disadvantages of various types, including the ability to readily convert the constituent polymers into hollow fibers, having desirable properties of selective permeability and flux therethrough.
One such material, otherwise a very efficient polymer membrane for many separation uses is low density polyethylene. In flat sheet form it performs well in many uses, but when converted to hollow fibers by spinning and drawing it loses many of its advantages. For example, permeability is severely restricted by the usual and well-known spinning processes, either melt or solvent. It is theorized that this reduction occurs because of much higher crystallinity induced by the stresses of spinning and processing.
Furthermore, many of the materials from which hollow fibers are formed undergo deterioration of permeability during use and even when stored under various protective media. This deterioration of permeability adversely affects these polymer materials during periods of extended use or extended storage. For example, separations of phenols from aqueous solutions require hollow fiber membranes of a combination of properties. These include selective permeability to organic compounds such as phenols, retention of such permeability over extended periods of storage and use and reasonable strength to maintain structural integrity of the relatively thin wall hollow fibers produced.